The present invention relates to a process for preparing 3-hydroxy-7-mercapto-pyrazolo[4,3-d]pyrimidines (which are useful as a starting material for preparation of pyrazolo[4,3-d]pyrimidine derivatives useful as an antihyperlipidemic agent as described hereinafter) from 3-hydroxy-4-nitroso-5-alkoxycarbonyl-pyrazole through a novel compound, 3-hydroxy-4-dialkylaminomethyleneamino-5-cyanopyrazole, and also to the novel intermediate compounds.
As described in European Patent Application No. 0157415 publised on Oct. 9, 1985, 3-hydroxy-7-mercaptopyrazolo[4,3-d]pyrimidine is useful as an intermediate product for the preparation of anti-hyperlipidemic agents. Heretofore, 3-hydroxy-7-mercapto-pyrozolo[4,3-d]pyrimidines (IX) have been prepared according to the following Reaction Scheme 1. ##STR4##
In the above formulae, R.sup.1 is a hydrogen atom, a lower alkyl group having 1 to 4 carbon atoms, a phenyl group or a substituted phenyl group substituted with an alkyl group having 1 to 4 carbon atoms or an alkyloxy group having 1 to 4 carbon atoms, and R.sup.3 is an alkyl group having 1 to 4 carbon atoms.
As shown in the Reaction Scheme 1, 3-hydroxy-4-nitroso-5-alkoxycarbonylpyrazole (II) is reduced and then cyclized in formamide by heating at elevated temperature to once form 3,7-dihydroxy-pyrazolo[4,3-d]pyrimidine (VIII) and, thereafter, this 3,7-dihydroxy-pyrazolo[4,3-d]pyrimidine (VIII) is heated in pyridine along with diphosphorus pentasulfide.
The above process has disadvantages in that heating at temperatures as high as more than 150.degree. C. is needed in the cyclization reaction to form 3,7-dihydroxy-pyrazolo [4,3-d]pyrimidine (VIII); the reaction between 3,7-dihydroxypyrazolo[4,3-d]pyrimidine (VIII) and diphosphorus pentasulfide is difficult to control and its yield is low; and in that the purity of the formed 3-hydroxy-7-mercaptopyrazolo[4,3-d]pyrimidine (IX) is low.